This invention relates to control circuits for solid state switches and, more particularly, to a control circuit which is isolated from the alternating current (AC) line which the switch controls.
In designing control systems for AC switches, transfer of control commands from a controller to the AC switches is a common design problem. Whenever solid state switches such as silicon controlled rectifiers (SCRs), triacs, or power transistors are used, the switch control terminal is not isolated from the AC line and is therefore live. The controller, on the other hand, normally must be isolated from the AC line for safety reasons, and, in any event, is usually referenced to another line potential. Another common problem is that the drive or control terminals of solid state switches require low voltage power which must be made available to each switch.
Traditional solutions to these design problems involve the use of opto-couplers (which provide isolation) in combination with transformers, capacitors or various other types of dissipative circuitry (which carry power at the solid state switch level). As an alternative, individual high frequency transformers are used to both carry the control signal and provide the drive power. However, all of these solutions are expensive and complex and the former also suffers from limited reliability.
The problems existing with traditional solutions are even more acute when considered in connection with the design of appliance controllers. Such controllers are characterized by a high number of low power, low cost solid state switches, very stringent cost requirements, and high reliability standards. The existing solutions in this environment all fail to meet at least one of the requirements of an acceptable controller.